What’s the difference between insects and arachnids?

Class insecta

Of the 1.5 million species that scientists have catalogued, around two thirds are insects.

Insects come in all shapes and sizes and display diverse behaviours. But they all share a set of defining features. What do ants, flies and beetles have in common? They all have antennae, 6 legs and bodies that consist of 3 parts: a head, a thorax and an abdomen.

They are also immensely successful. Take ants, for example. Famous biologists predict that there are 10 quadrillion of them. That’s 10,000,000,000,000,000…or around 1.4 million ants for every human on the planet!

Class arachnida

Arachnids get their name from Arachne, the shepherd’s daughter and master weaver from Greek mythology, who was later turned into a spider. Indeed, spiders are the largest order in this class. Other arachnids include scorpions, ticks, mites and solifugae (also known as camel spiders, wind scorpions or sun spiders).

They often get lumped together with insects as ‘bugs’. I dislike the imprecision of this term, but the two classes are similar in many respects. You see, insects and arachnids are both arthropods. They’re invertebrates with exoskeletons, jointed legs and segmented bodies.

The difference lies in the numbers. Arachnids have 8 legs and there are just 2 segments to their bodies. Their head and thorax are combined, forming something called a cephalothorax.

Close up of middle east wind scorpion on a white background. Pic by Dmitry Abezgauz | Shutterstock.com
Close up of middle east wind scorpion on a white background. Pic by Dmitry Abezgauz | Shutterstock.com

Praying mantis

Let’s kick off with perhaps the best adapted predator of the insect world: the praying mantis. Making the most of camouflage and their plant-like structure, praying mantids are the masters of ambush.

They may be small but they’ll attack pretty much anything that comes within reach of their spiked, raptorial legs. We’re not just talking about other insects (like the bees in the video below), they also prey on small rodents, frogs, lizards and even small birds!

Besides stealth, these formidable hunters have incredible vision. Not only do they have two large compound eyes on the sides of their head, they also have three more eyes in between. Mantids are even capable of rotating their heads 180 degrees to expand their visual field. No other order of insect can do this.

What adds to their insatiable reputation is their participation in sexual cannibalism. Never ones to miss a meal, female mantids are known to eat their male counterparts alive, during or shortly after mating.

Brazilian Wandering Spider

The Brazilian wandering spider is in fact the name of two distinct species, Phoneutria fera and Phoneutria nigriventer. Sometimes they are also referred to as banana spiders, due to their well-documented knack of turning up in people’s fruit bowls!

Why should that matter? Well, both species comfortably make any world’s deadliest spiders list. Their venom is highly toxic, but the severity of its effect depends on how much venom has been delivered. Generally, female Brazilian wandering spiders produce a higher quantity of venom than males.

Their venom attacks the nervous system, causing excruciating pain, loss of muscle control, breathing problems. In male victims, a bite can lead to priapism, a prolonged, painful penile erection. If that wasn’t bad enough, at deadly concentrations, the venom can result in paralysis and asphyxiation.

Venomous wandering spider (Phoneutria sp.), Ecuador. Pic by Dr Morley Read | Shutterstock.com
Venomous wandering spider (Phoneutria sp.), Ecuador. Pic by Dr Morley Read | Shutterstock.com

Huntsman spider

Next up is a family of truly enormous arachnids. There are 83 species of huntsman spider, including the largest spider in the world, the Australian Heteropoda maxima, which has a 30cm leg-span. While they’re often referred to as tarantulas, they actually belong to the family Sparassidae.

Huntsman spiders are nowhere near as venomous as Brazilian wandering spiders, nor are they particularly aggressive. While their fangs are certainly long and sharp enough to pierce human skin, a huntsman spider bite is unlikely to lead to anything more than pain and swelling.

Even if they don’t pose a mortal threat to humans, these highly effective predators are well worth their inclusion here. As their name would suggest, instead of spinning webs and seeing what comes along, huntsman spiders actively stalk their prey.

What makes them so effective? To start with, they’re remarkably quick. It’s not easy to measure the speed of a spider, but it’s been estimated that the fastest huntsman species can run 42 body lengths per second.

Not only are they fast runners, but they are also impressively powerful. Check out this one dragging a mouse up the side of a fridge!

Epomis ground beetle

All of the invertebrates we’ve seen so far are capable of taking down prey at least twice their size, but none disrupt the natural order of things quite like Epomis ground beetle larvae. These white grubs, measuring no more than 20mm, feed exclusively on amphibians.

It’s a rare reversal of the predator-prey relationship. You can think of it like David and Goliath, only that in this case David kills and eats giants on a regular basis!

How do they do it? As you can see from the video, the larva lures the frog, simply by resembling prey. When the frog strikes, the larva evades the attack. Prey turns predator and the larva bites the throat or underside of the amphibian with its double-hooked mandibles. Parasitic behaviour ensues as the larva attaches to the frog and starts to eat it.

There’s something unsettling about this role reversal, but it’s certainly a fascinating adaptation.


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James is a freelance language professional based in Barcelona. Besides writing articles for Zoo Portraits, he writes about education and teaches English as a foreign language in businesses, universities and other institutions around the city.


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